Cold stratification is a plant defense mechanism, designed to keep seeds from breaking dormancy and sprouting at the wrong time. In nature, seeds would cold stratify all on their own under a blanket of snow. Starting seeds for your garden in the spring is a different matter, as those seeds have likely been in a warm, room temperature packet all winter long. No worries, it’s easy enough to stratify seeds at home before planting.
I spend a lot of the winter months here in Vermont flipping through garden catalogs and dreaming of my spring garden. They’re full of beautiful pictures and text designed to convince you to grow just about everything under the sun. The problem is, when those little seed packets arrive there’s often an extra step before planting: Cold Stratification.
I’ve been caught off guard by the need to cold stratify seeds more than once, and now I look over the planting instructions on every single packet as soon as they arrive. The catalogs always gloss over the technical details of growing particular crops, but the seed packets are a wealth of information. Take a look at the packets, and they’ll say something like “Stratify seeds for 6 weeks before planting” or “seeds require 5 days cold, moist stratification to break dormancy.”
The amount of time will depend on the crop, but for the most part, the process of stratifying seeds is the same.
How to Stratify seeds
The most dependable way to stratify seeds is in a moist medium, wrapped in plastic in the refrigerator. Larger seeds tend to do well in a bit of moistened peat or sand, placed into a plastic bag. Smaller seeds can be distributed onto moist paper towels. The trick is to keep them moist, but not sopping wet.
The bag itself is designed to retain moisture, but like all living things, the seeds need to breathe. Ensure adequate ventilation during cold stratification by cracking the top of the bag open, or punching a few small holes into the bag. I generally use Ziploc storage bags, and I just don’t seal the top.
Some instructions will tell you to moisten the medium with a bit of water and hydrogen peroxide, and the added peroxide will sterilize the medium a bit and prevent mold. I haven’t found this necessary, and if you find yourself battling mold consider adding less water to the towels, increasing ventilation or moving them to a colder spot in the fridge (I keep them at the way back).
How Long to Cold Stratify Seeds
The length of time to cold stratify seeds really depends on the variety. I’ve seen packets labeled “cold stratify for 5 days” and I wonder, what’s the point? But that plant evolved that response for a reason and the times I’ve ignored it they don’t sprout. Most plants that require stratification need a bit longer than that, usually at least 4 to 6 weeks.
As a general rule, the hardier the plant, the longer they need to cold stratify. Sea Buckthorn, a very hardy Siberian superfruit that we grow here on our homestead requires 90 days of cold moist stratification. It’s hardy down to zone 2, and it’s hard to find a corner of the earth too cold for this resilient plant.
They’re a bit tricky beyond the cold stratification, as this plant also requires scarification to germinate. That’s another plant defense mechanism, designed to keep seeds that fall from the mother plant from germinating and overcrowding. The seeds will only sprout if they’re beaten up a bit, as though they’ve gone through a bird’s acidic digestive system as they were carried far away from the parent plant. You can accomplish this by rubbing them between 2 sheets of coarse sandpaper to rough them up a bit BEFORE cold stratification.
Growing apples from seed, on the other hand, is a bit simpler. They generally grow from zones 4 to 7/8 and most varieties require about 6 weeks of cold stratification. Actually just storing the apples in the refrigerator often does the trick, and if you’re buying apples late in the winter they’ve likely already spent weeks in cold storage before they hit the grocery store shelves.
On occasion, I’ve even cut into apples that had seeds already sprouting inside as a result of long cold moist stratification more or less by accident in cold storage. For the most part though, if you place apple seeds on moist paper towels in the fridge they’ll start to germinate right around the 6-week mark.
Lazy Seed Stratification
While most plants respond well to cold, moist stratification using sand, peat or paper towels…others are less picky. Cold dry stratification works well with some varieties, and that basically means just sticking the whole seed packet in the back of the refrigerator.
I’ve had pretty good success with this method in the past few years, as two toddlers running around means I have A LOT less time to futz around with the pickiest of plants. When those surprise packets arrive with the words “cold stratify” on the outside, I just stick them right in the fridge and it’s worked out pretty well thus far.
We have a huge bumper crop of marshmallow plants as a result of that lazy method, no paper towels required.
Cold Stratification In the Garden
Another very simple method for cold stratification is to simply plant the seeds outdoors late in the fall. This mimics nature’s natural rhythms, and it’s like the plants just dropped their seed heads at the end of the season. The seeds will overwinter under a blanket of snow, staying just the right temperature for germination.
A couple of things to keep in mind with this method though…
- The seeds are outdoors, and could just as easily be eaten by moles, chipmunks or birds. That’s one reason to plant them as late in the fall as possible, to minimize the time they’re out there before snow cover protects them (somewhat).
- This only works if you’re in the right growing zone. Chokecherries, for example, will grow as bushes as warm as zone 8. The seeds, however, won’t cold stratify in that climate. They’ll only grow from seed outdoors in cooler climates, below zone 6. If you’re in a warm climate, keep in mind you may need to artificially stratify seeds.
Seeds that Don’t Require Cold Stratification
When I learned about cold stratification, I assumed that most plants, especially perennials, grown in our cold northern climate would require cold stratification. Not the case. When I was researching how to grow rhubarb from seed I was pleasantly surprised that it doesn’t require cold stratification.
That was great news, as it was spring already and those seeds had been stored all winter in my pantry…
I also learned that it’s actually grown as an annual in hot climates (zone 9/10) by direct seeding fresh seeds in the fall, and growing the cold-loving plants in a hot climate winter. More good news, since my family loves rhubarb and lives in the Mojave desert!
Similarly, you can grow strawberries from seed without any cold stratification. It just goes to show you that there’s really no telling what’s going to require cold stratification. As a rule, many but not all, native plants from cold climates will require cold stratification. Good examples include common milkweed, St. Johns Wort, coneflowers, and rudbeckia.
See, that seed packet surprise didn’t turn out so bad after all! You learned a few new things about your plants and got to participate in a time-honored winter ritual that the plants have been enduring year after year as a part of their natural cycle of life. Isn’t it neat that this time it happened right in your own fridge?